1. Field of the Invention
This invention relates to composite rubber bodies reinforced with steel cords for rubber articles such as pneumatic tires, conveyer belts and the like. More particularly it relates to a composite rubber body attaining reduction of weight and improvement of durability by using steel cords having a high tensile strength and an excellent ductility.
2. Description of the Related Art
In order to reduce the weight of the composite rubber body reinforced with steel cords, it is advantageous to increase the tensile strength of the steel cord constituting the composite rubber body so as to decrease the amount of steel cords used in the composite rubber body with less or thinner steel cords. In order to improve the durability of the composite rubber body, it is advantageous to improve the fatigue resistance of the steel cord. For this end, it is more strongly demanded to improve the tensile strength and fatigue resistance of the steel cord. Further, in order to economically produce such steel cords, it is important to ensure the ductility of each steel filament wire constituting the steel cord to prevent the occurrence of breakage during twisting.
As a technique for improving the tensile strength of the steel cord, there are proposed a technique wherein a carbon content of a wire rod as a starting material for the steel cord is made higher than that of an ordinary wire rod, a technique wherein a wire drawing ratio of the wire rod is increased, and the like. However, the increase of the carbon content or the wire drawing ratio in the wire rod produces a new problem of degrading the fatigue resistance of the steel cord. For this end, in order to improve the fatigue resistance of the steel cord, it is attempted to add new element(s) to the composition of the wire rod as a starting material for the steel cord to form a low alloy steel having a fine pearlite structure, or to decrease the amount of non-metal inclusion in the wire rod, or the like.
JP-A-60-152659 proposes a wire rod for fine wire comprising C: 0.60-0.90 wt %, Si: 0.10-0.50 wt %, Mn: 0.30-1.00 wt %, P: less than 0.10 wt %, S: less than 0.005 wt %, Cu+Ni+Cr&lt;0.10 wt %, total oxygen: not more than 30 ppm and total nitrogen: not more than 30 ppm and having an excellent processability. In Examples thereof, filament wires having a tensile strength of 374-381 kgf/mm.sup.2 and a reduction of area of 34-37% are obtained by drawing a wire rod of 1.62 mm in diameter to a diameter of 0.23 mm.
JP-A-1-226404 proposes a method of applying steel cords of open structure obtained by 3-5 high-strength steel filament wires having a diameter of 0.13-0.32 mm to a carcass of a tire in order to largely reduce the tire weight and improve the durable life of the tire.
However, expensive elements are used in the low alloy steel, so that the cost of the starting material for the steel cord undesirably rises. On the other hand, when the non-metal inclusion is decreased in the wire rod as far as possible, the production steps become complicated to increase the production cost. In any case, these techniques are disadvantageous in view of economical reasons. Even when such a wire rod is used to produce steel cords, the high tensile strength and good fatigue resistance are not necessarily satisfied at the same time. Especially, the degradation of the fatigue resistance is a serious problem in the rubber articles such as tires and the like subjected to repetitive bending under loading because some of steel filament wires constituting the steel cord are broken by the repetitive bending to bring about the occurrence of cord breakage and hence result in fatigue breakage of the composite rubber body or the tire.
JP-A-60-152659 mentions the tensile strength and reduction of area in the filament wire after the drawing but does not mention the influence by vulcanization at the step of forming the composite rubber body at all, so that there is caused a problem of age embrittlement.
JP-A-1-226404 defines the tensile strength of the filament wire but does not mention the fatigue property of the filament wire at all. Moreover, the resistance to corrosion fatigue in the steel cord of open structure is improved, but there is no description on the mechanical fatigue property.